The present disclosure relates to a sheet folding apparatus which folds sheets of paper in two and a post-processing apparatus which includes the sheet folding apparatus.
At present, a sheet post-processing apparatus which has an apparatus which staples or punches sheets on which images are formed by an image forming apparatus, or a sheet folding apparatus which folds such sheets in two is spread.
In general, a sheet folding apparatus which folds sheets in two has a couple of two-folding rollers and a folding plate which can move forward to or backward from a nip between the two-folding rollers. The sheet folding apparatus folds the sheets by pushing the sheets upward with the folding plate so as to insert the sheets between the two-folding rollers and pressing the sheets between the two-folding rollers.
A roller with a column-shaped central shaft and a rubber layer surrounding the shaft is typically used as the two-folding roller. If the roller with such a construction is formed so as to have the uniform outer diameter in a longitudinal direction, the pressing force of each end part of the roller in a longitudinal direction tends to be stronger than that of the central part of the roller because of the deformation of the roller and the like when the two rollers are pressed each other. This causes the problem that force is applied to the sheets from the outer side to the inner side in a width direction when the sheets passes between the rollers, so that the sheets are creased.
In consideration of this, a two-folding roller whose outer diameter is set so as to gradually decrease from the central part to each end part in an axial direction is proposed. In the roller with such a shape, the pressing force is higher at the central part having the large diameter when the two rollers are pressed each other, and the above-described tendency is cancelled each other out, and therefore, a uniform pressing force can be obtained in an axial direction in the rollers.
In a sheet folding apparatus with such two-folding rollers, in the case where the edge of the folding plate does not go over the nip between the two-folding rollers when the sheets are folded in two, the bulging edge part of the folded sheets, which is formed when the sheets are pushed upward and are folded with the folding plate, cannot go over the nip. As a result, the sheets are pushed back from the nip, or, the most outer sheet goes over the nip, and thus, a paper jam occurs, or, if the sheets are stapled, the sheet is torn.
In the case where the edge of the folding plate goes over the nip between the two-folding rollers, the sheets are not pushed back. However, when the folding plate is pulled out from the nip, the friction between the folding plate and the most inner sheet increases between the central parts of the two-folding rollers each having a large diameter, and, as a result, gloss can be produced on the sheet and it causes defects in the images, or the most inner sheet may be pulled back together with the folding plate, so that a paper jam occurs, or the sheet is torn.
In consideration of this, a post-processing apparatus in which a cutout part which is not inserted into the nip is formed in the part of the edge of the folding plate corresponding to the central parts of the two-folding rollers each having a large diameter is proposed.
However, in the post-processing apparatus in which the cutout part is formed in the edge of the folding plate, when one sheet is set between the two-folding rollers and the folding plate, the central part of the sheet is placed on the edge of the folding plate, and, at this time, the part of the sheet corresponding to the cutout part can bend down because of its weight. If the sheet is inserted into the nip between the two-folding rollers with the folding plate in the state where the sheet is bending down, the bent part of the sheet is creased.
If a projected part is formed in the cutout part to prevent the crease of the sheet, the sheet can be set without bending down, and cannot be creased. However, since the projected part is inserted into the nip, gloss can be produced in the part of the sheet corresponding to the projective part when the folding plate is pulled out from the nip, as described above.